Measurements of Ep in homogenates of kidneys will potentially improve our understanding of factors that influence renal Ep production. Until now studies have shown that Ep appears in the kidney prior to the plasma during exposure to hypoxia, and disappears from the kidneys before the plasma after cessation of hypoxia; that the medullae contains less Ep than do the renal cortices; and that male kidneys have higher titers of Ep/gm tissue then do those of females after exposure of rats to hypoxia. Also injection of Ep in large titers appears to inhibit the release of Ep from the kidneys but doesn't directly influence renal Ep production. We intend to study effects of other conditions and substances on renal and plasma Ep titers; to determine hepatic and renal Ep titers after exposure of rats of various ages to hypoxia and to pursue further studies on factors that influence extrarenal Ep production.